Content addressable memory (CAM) devices, sometimes referred to as “associative memories,” can receive a compare data value (in some cases referred to as a comparand or search key), and compare such a value against a number of stored data values. In most configurations, such an operation can match a compare data value against a very larger number of stored data values (e.g., thousands or millions), essentially simultaneously.
Such rapid compare functions have resulted in CAM devices enjoying wide application in various packet processing hardware devices, such as routers and network switches, to name just two. In a typical packet processing operation, a device can receive a packet. The packet can include various data fields that indicate how the packet should be processed or what the data packet contains. A hardware device can use a matching function, provided by a CAM device, to compare one or more data fields to stored data values and thereby indicate how the packet is to be processed.
Many CAM device configurations can include a number of CAM memory cells arranged into an array of rows and columns. CAM cells of a same row can be commonly connected to a match line, that can provide a match result representing a comparison between data values stored in the row, and a received compare data value. CAM cells of a same column can be commonly connected to a bit line pair, in order to read and/or write data to CAM cells of the column. Similarly, CAM cells of a same column can be commonly connected to compare data lines that can provide a compare data value to the CAM cells of the column.
To better understand various aspects of the embodiments, a conventional CAM device circuit will now be described.
Referring now to FIG. 26A, an example of a conventional CAM device is shown in a top plan view and designated by the general reference character 2600. A CAM device 2600 can include a substrate 2602 in which can be formed a number of CAM cells. Areas of a substrate occupied by each CAM cell are shown as 2604-0 to 2604-5. FIG. 26A also shows interconnection metallization line 2606 that is formed over substrate 2602 and can includes contacts 2608-0 to 2608-5 to CAM cell areas (2604-0 to 2604-5). A metallization line 2606 can provide a common connection to CAM cells of a CAM device, such as a match line. Successive compare operations typically result in a match line being continuously precharged and then discharged (in the event of a non-match result).
Referring now to FIG. 26B, a side cross sectional view taken along line B-B of FIG. 26A shows metallization line 2606 extending to diffusion regions (one shown as 2610) formed in a p-type substrate 2612.
It is understood that FIGS. 26A and 26B are not drawn to scale, and are intended to show the general placement of structures in one conventional arrangement.
Referring now to FIG. 27, a CAM device, like that shown in FIGS. 26A and 26B is shown in a diagram that illustrates capacitive effects on a match line. FIG. 27 shows a CAM device 2700 having a first match lines 2702-0 and an adjacent second match line 2702-1. Match lines (2702-0 and 2702-1) can be formed by patterning a first metallization layer formed over a substrate. Each match line (2702-0 and 2702-1) has a direct connection to transistors within compare circuits of CAM cells of the same row. This is represented in FIG. 27, by match line 2702-0 being connected to a drain of transistor 2704.
A structure of CAM device 2700 can result in various components contributing the effective capacitance of a match line (2702-0 and 2702-1). FIG. 27 shows such capacitive components for first match line 2702-0 as capacitances Cdiff, Cplate, CML, CM2, CM3. Capacitance Cdiff includes a diffusion capacitance formed at the multiple drain connections to match line 2702-0. Capacitance Cplate includes a capacitance presented by the structure of match line 2702-0 itself as a metal “plate” separated from a substrate by insulating layers. Capacitance CML can be a capacitance due to coupling between match line 2702-0 and adjacent match line 2702-1 (as well as any other adjacent match lines). Capacitance CM2 can be a capacitance due to coupling between match line 2702-0 and an overlying second metallization layer 2706. Similarly, capacitance CM3 can be a capacitance due to coupling between match line 2702-0 and an overlying third metallization layer 2708.
As in the case of FIGS. 26A and 26B, the structures of FIG. 27 are not drawn to scale, and are intended to represent a general relationship between the structures. If capacitances CML, CM2, CM3 are commonly grouped into a single value Ccouple, a capacitance for match line 2702-0 can be given asCtot=Cdiff+Cplate+Ccouple.As noted above, in operation, match lines are typically charged and discharged repeatedly. Keeping in mind the above relationship, a match line power consumption (P) can be proportional to such a capacitance, as shown by the relationship:P∝Ctot*V2*f where V is the switching range voltage for the match line, and f is the frequency of switching. A typical CAM device can include thousands of match lines, thus match line switching (charging and discharging) can be a considerable source of power consumption.
At the same time, the speed by which a match line can be charged can be represented by a time constant (τ) for a match line:τ=RML*CtOt where RML can be a unit resistance for a match line.
Conventionally, to provide a low value of RML for a match line structure, match lines have been formed with a metallization layer.